IrO2 catalysts and methods of use thereof

ABSTRACT

Embodiments of the present disclosure provide for IrO 2  catalysts, methods of making IrO 2  catalysts, methods of using IrO 2  catalysts to make methanol, formaldehyde, and/or ethylene from CH 4 , systems for using IrO 2  catalysts, and the like.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. Provisional Application Ser. No. 62/479,081, having the title “METHANE ACTIVATION ON THE IrO₂(110) SURFACE”, filed on Mar. 30, 2017, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND

The increasing supply of natural gas provides substantial motivation for developing catalytic processes that can efficiently and directly transform methane (CH₄) to value-added products such as methanol, formaldehyde, or ethylene. Selective catalytic transformations of CH₄ remains a major challenge in catalysis.

SUMMARY

Embodiments of the present disclosure provide for IrO₂ catalysts, methods of making IrO₂ catalysts, methods of using IrO₂ catalysts to make methanol, formaldehyde, and/or ethylene from CH₄, systems for using IrO₂ catalysts, and the like.

In an aspect, the present disclosure provides for a catalyst comprising: a IrO₂(110) substrate having a rutile IrO₂(110) surface having exposed cus-Ir atom sites. In an embodiment, the stoichiometric termination of rutile IrO₂(110) has a rectangular unit cell with dimensions of 3.16×6.36 Å with the corresponding lattice vectors aligned along the [001] and [110] crystallographic directions, respectively.

In an aspect, the present disclosure provides for a method of making an IrO₂(110) surface, comprising: oxidizing Ir(100) substrate at about 725 K to 875K and a O₂ partial pressure of about 0.5 to 100 Torr; evacuating O₂ from the chamber until the pressure reaches less than 10⁻⁷ Torr with the sample held at 600 to 650 K; and forming a rutile IrO₂(110) surface having exposed cus-Ir atom sites.

In an aspect, the present disclosure provides for a method of making a product from CH₄, comprising: exposing a catalyst comprising an IrO₂ substrate having a rutile IrO₂(110) surface having exposed cus-Ir atom sites and a CH₄ gas to one another; and forming one or more products. In an aspect, the products can include CH₃OH, CH₂O, C₂H₄, or a combination thereof.

In an aspect, the present disclosure provides for a system of activating CH₄, comprising: a first device for introducing CH₄ to a catalyst comprising an IrO₂ substrate having a rutile IrO₂(110) surface having exposed cus-Ir atom sites; a second device for collecting one or more products of the catalytic reaction of CH₄.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aspects of the present disclosure will be more readily appreciated upon review of the detailed description of its various embodiments, described below, when taken in conjunction with the accompanying drawings.

FIGS. 1A-B provide example characterizations of the IrO₂(110) surface.

FIGS. 2A-B are examples of TPRS spectra showing the adsorption and reaction of CH₄ on IrO₂(110). TPRS yields are shown in FIGS. 2C-2D.

FIGS. 3A-C are examples of kinetic analysis of CH₄ dissociation on the IrO₂(110) surface. FIG. 3A shows reaction yield vs. CH₄ exposure to IrO₂(110) for different surface temperatures. FIG. 3B provides the initial dissociation probability vs. surface temperature and FIG. 3C is an Arrhenius plot as discussed in the text.

FIG. 4 provides example energy diagrams for the formation and C—H bond cleavage of a CH₄ σ-complex on IrO₂(110).

FIGS. 5A-B provide an estimation of IrO₂(110) film thickness.

FIGS. 6A-B provide examples of adsorption of O₂ and H₂O on IrO₂(110). The TPD spectrum of O₂ (FIG. 6A) and H₂O (FIG. 6B) was taken after exposing the IrO₂(110) film to 10 L O₂ and 1.5 L H₂O at 88 K, respectively.

FIGS. 7A-C provide examples of adsorption and reaction of CD₄ on IrO₂(110).

FIG. 8 is an example of a charge-density difference plot (electrons/bohr³) for the η² CH₄ σ-complex on IrO₂(110) predicted using DFT-PBE viewed along the [001] direction.

The drawings illustrate only example embodiments and are therefore not to be considered limiting of the scope described herein, as other equally effective embodiments are within the scope and spirit of this disclosure.

DETAILED DESCRIPTION

Before the present disclosure is described in greater detail, it is to be understood that this disclosure is not limited to particular embodiments described, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present disclosure will be limited only by the appended claims.

Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit (unless the context clearly dictates otherwise), between the upper and lower limit of that range, and any other stated or intervening value in that stated range, is encompassed within the disclosure. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges and are also encompassed within the disclosure, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the disclosure.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present disclosure, the preferred methods and materials are now described.

As will be apparent to those of skill in the art upon reading this disclosure, each of the individual embodiments described and illustrated herein has discrete components and features which may be readily separated from or combined with the features of any of the other several embodiments without departing from the scope or spirit of the present disclosure. Any recited method can be carried out in the order of events recited or in any other order that is logically possible.

Embodiments of the present disclosure will employ, unless otherwise indicated, techniques of chemistry, inorganic chemistry, material science, and the like, which are within the skill of the art. Such techniques are explained fully in the literature.

The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how to perform the methods and use the compositions and compounds disclosed and claimed herein. Efforts have been made to ensure accuracy with respect to numbers (e.g., amounts, temperature, etc.), but some errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, temperature is in ° C., and pressure is in atmosphere. Standard temperature and pressure are defined as 25° C. and 1 atmosphere.

Before the embodiments of the present disclosure are described in detail, it is to be understood that, unless otherwise indicated, the present disclosure is not limited to particular materials, reagents, reaction materials, manufacturing processes, or the like, as such can vary. It is also to be understood that the terminology used herein is for purposes of describing particular embodiments only, and is not intended to be limiting. It is also possible in the present disclosure that steps can be executed in different sequence where this is logically possible.

It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a support” includes a plurality of supports. In this specification and in the claims that follow, reference will be made to a number of terms that shall be defined to have the following meanings unless a contrary intention is apparent.

Discussion

Embodiments of the present disclosure provide for IrO₂ catalysts, methods of making IrO₂ catalysts, methods of using IrO₂ catalysts to make methanol, formaldehyde, and/or ethylene from CH₄, systems for using IrO₂ catalysts, and the like. IrO₂ catalyst of the present disclosure are advantageous because they facilitate the adsorption and C—H bond activation of CH₄. CH₄ readily undergoes C—H bond cleavage on the IrO₂ catalyst (e.g., IrO₂(110) surface) at temperatures of about 150 K. The initial dissociation of CH₄ on IrO₂ catalyst occurs through a precursor-mediated process where the activation energy for initial C—H bond cleavage is 9.5 kJ/mol lower than the binding energy of the molecularly adsorbed precursor.

In an aspect, the present disclosure provides for a IrO₂ catalyst having a IrO₂(110) substrate having a rutile IrO₂(110) surface having exposed cus-Ir atom sites. The stoichiometric termination of rutile IrO₂(110) has a rectangular unit cell with dimensions of about 3.16×6.36 Å with the corresponding lattice vectors aligned along the [001] and [110] crystallographic directions. Rows of cus-Ir atoms (Ir_(cus)) are separated by rows of bridging-O atoms (O_(br)) that run parallel to the [001] direction. The Ir_(cus) and O_(br) atoms each lack a bonding partner compared with the bulk and expose single coordination vacancies. Based the IrO₂(110) unit cell, the areal densities of Ir_(cus) atoms and O_(br) atoms would each equal to about 34 to 40, about 37%, or 37% of the surface atom density of Ir(100). Because the cus-metal atoms are active adsorption sites, adsorbate coverages were specified in units of ML (monolayer), where 1 ML is equal to the density of Ir_(cus) atoms on the IrO₂(110) surface.

In an aspect, the IrO₂ catalyst can be a layer on another substrate or can be the substrate itself. The IrO₂ catalyst can be a particle having dimensions in the micrometer to nanometer range and can have regular (e.g., spherical) or irregular shapes. In embodiments where the IrO₂ catalyst is a layer, the layer can be of a thickness sufficient to achieve activation of the C—H bonds and can be in the monolayer to nanometer range or more.

In an embodiment of the present disclosure, the IrO₂ catalyst can be made by oxidizing an Ir(100) substrate at about 725 to 875 K and a O₂ partial pressure of about 0.5 to 100 Torr for a time period of about 5 to 20 minutes. The O₂ gas is subsequently evacuated from the reaction vessel with the sample held at a temperature between 600 and 650 K. After the pressure falls below 10⁻⁷ Torr, the reactive IrO₂(110) layer may be cooled to lower temperature to preserve the reactive cus-surface sites. Additional details are provided in Example 1.

The IrO₂ catalyst can be used to produce desired products due to its ability to activate C—H bonds in CH₄. In an aspect, the method of making a product from CH₄ can include exposing the IrO₂ catalyst comprising an IrO₂ substrate having a rutile IrO₂(110) surface having exposed cus-Ir atom sites and a CH₄ gas to one another, where the catalytic reaction of the gas with the IrO₂ catalyst forms one or more products. The products formed can include: CH₃OH, CH₂O, C₂H₄, and a combination thereof.

A system can be used to conduct the catalytic reaction of the IrO₂ catalyst and the CH₄ gas. The system can include a first device (e.g., a reaction chamber made of a material such as stainless steel) that can include the IrO₂ catalyst. The temperature and pressure of the reaction chamber can be controlled using known vacuum systems and temperature control systems. The CH₄ gas can be introduced to the reaction chamber and the temperature and the pressure can be adjusted to produce desired product(s) (e.g., CH₃OH, CH₂O, C₂H₄, and a combination thereof). The CH₄ readily undergoes C—H bond cleavage on the IrO₂(110) surface through a precursor-mediated process. Additional reactants can be added before, during and/or after the catalytic reaction to produce the desired product(s). The system includes a second device (e.g., another chamber made of a material such as stainless steel or other appropriate material) that is part of or interfaced with the first device to separate, remove, or capture the desired product(s) using known vacuum technologies, gas separation or capture technologies, and the like. Once the desired product(s) are obtained, they can be appropriately processed for future use. The system can be configured to process CH₄ gas in a systematic manner that maximizes the life of the IrO₂ catalyst.

EXAMPLES

Now having described the embodiments of the disclosure, in general, the examples describe some additional embodiments. While embodiments of the present disclosure are described in connection with the example and the corresponding text and figures, there is no intent to limit embodiments of the disclosure to these descriptions. On the contrary, the intent is to cover all alternatives, modifications, and equivalents included within the spirit and scope of embodiments of the present disclosure.

Example 1

A limitation with most existing heterogeneous catalysts is that initial C—H bond cleavage is rate-controlling, so subsequent reaction steps occur rapidly and are difficult to control. Achieving CH₄ activation at low temperature could eliminate this limitation and allow for its selective oxidation. However, catalytic materials that can readily activate CH₄ at low temperatures (e.g., below 300 K) have not been reported.

The activation of light alkanes on solid surfaces can occur by direct and precursor-mediated mechanisms. In the direct mechanism, the alkane molecule undergoes C—H bond cleavage during its initial collision with the surface and reaction is activated with respect to the gas-phase energy level. In the precursor-mediated mechanism, the alkane first adsorbs intact on the surface and the resulting molecularly adsorbed state serves as a precursor for C—H bond cleavage. Dissociation by the precursor-mediated mechanism is facile when the activation energy for C—H bond cleavage (E_(r)) is smaller than the activation energy for desorption (E_(d)) of the molecularly adsorbed precursor. Molecular beam experiments show that CH₄ dissociation is activated (E_(r)>E_(d)) on the many crystalline transition-metal surfaces that have been investigated. Facile dissociation (E_(r)<E_(d)) of CH₄ on a solid surface has not been previously reported, but other light alkanes do undergo facile activation on certain facets of metallic Ir and Pt. Prior studies also report only weak molecular adsorption of alkanes on many metal oxides, including alkaline-earth oxides, rare-earth oxides and TiO₂.

Previous studies indicate that specific facets of late transition-metal (TM) oxides, in particular PdO(101), can promote alkane C—H bond cleavage (11, 12). The key aspect of these surfaces is the presence of pairs of coordinatively unsaturated (cus) metal and oxygen atoms on the surface that promote the formation and facile C—H bond cleavage of adsorbed alkane σ-complexes (11). The cus-Pd sites of PdO(101) datively bond with alkanes (11-13), and that the resulting molecularly-adsorbed species are analogous to coordination compounds known as alkane σ-complexes (14). The dative interaction with cus-metal sites facilitates alkane activation by both strengthening the alkane-surface binding as well as weakening the Pd-coordinated C—H bonds. The cus-oxygen atoms also play a central role in alkane C—H bond cleavage on PdO(101) by acting as H-atom acceptors. In situ measurements show that formation of a PdO(101) layer gives rise to high rates of CH₄ oxidation over Pd surfaces under steady-state conditions at elevated pressure, thus demonstrating that fundamental studies with PdO(101) are directly relevant for understanding CH₄ oxidation over Pd surfaces under realistic conditions (15).

Density functional theory (DFT) calculations predict that small alkanes also form strongly-bound σ-complexes on rutile RuO₂ and IrO₂ surfaces (11, 16-20). The formation of alkane σ-complexes on RuO₂(110) has been confirmed, and it has also been shown that n-butane undergoes facile C—H bond cleavage during temperature-programmed reaction spectroscopy (TPRS) experiments in ultrahigh vacuum (UHV) (19, 20). Dispersion-corrected DFT calculations predict that the binding energy of the CH₄ σ-complex on IrO₂(110) is greater by about 40 kJ/mol than the energy barrier for C—H bond cleavage, so that CH₄ activation should occur at high rates on IrO₂(110) at temperatures as low as 100 K (17, 18).

The facile activation of CH₄ by the IrO₂(110) surface reinforces earlier studies which show that iridium possesses an unusual ability to activate hydrocarbon C—H bonds. As originally reported by Ardntsen and Bergman (21), cationic Ir(III) complexes are among the most highly reactive transition-metal compounds known for promoting C—H bond activation. Further, crystalline surfaces of metallic Ir exhibit the highest activity toward alkane C—H bond cleavage among the metal surfaces that have been investigated, and the presence of low coordination surface sites strongly enhances the reactivity of Ir and other metals toward alkane activation. A common feature among these systems is the availability of coordinatively-unsaturated Ir centers to bind and activate adsorbed alkanes.

Experimental reports of the growth and surface chemistry of crystalline IrO₂ are sparse because well-defined IrO₂ surfaces are challenging to prepare for fundamental UHV studies. At the O₂ partial pressures typically used in UHV experiments, oxygen adsorption on crystalline Ir surfaces reaches an effective saturation at submonolayer O-atom coverages (˜0.50 ML) because kinetic limitations suppress more extensive oxygen uptake. An in situ surface x-ray diffraction study shows that relatively thick layers of rutile IrO₂, exposing (110) and (100) facets, can form during Ir(111) oxidation but only when the O₂ partial pressure and temperature are >100 mbar and 775 K (23). Oxidation of Ir(111) with plasma-generated oxygen beams can produce multilayer IrO₂ structures under UHV conditions (24, 25). Rai et al. reported that a high-quality IrO₂(100) layer forms during Ir(111) oxidation with gaseous O-atoms, with the oxide saturating at a thickness of about four layers for growth temperatures below ˜650 K (25). However, the IrO₂(100) layer was completely oxygen-terminated, so this surface would be chemically inactive for CH₄ reactions at moderate temperature. Thus, the formation of relatively thick, rutile IrO₂(110) surfaces via metallic Ir oxidation that expose cus-Ir sites occurs only at sufficiently high temperature and requires relatively high oxidant fluxes.

In the present disclosure, investigations included the adsorption and C—H bond activation of CH₄ on a high-quality IrO₂(110) layer that was grown by oxidizing Ir(100) at 775 K and an O₂ partial pressure of 5 Torr. It is shown herein that CH₄ readily undergoes C—H bond cleavage on the IrO₂(110) surface at temperatures down to at least 150 K. Analysis of temperature-dependent rate data shows that the initial dissociation of CH₄ on IrO₂(110) occurs through a precursor-mediated process wherein the activation energy for initial C—H bond cleavage is 9.5 kJ/mol lower than the binding energy of the molecularly adsorbed precursor.

The stoichiometric termination of rutile IrO₂(110) has a rectangular unit cell with dimensions of (3.16×6.36 Å) with the corresponding lattice vectors aligned along the [001] and [110] crystallographic directions, respectively (FIG. 1A). Rows of cus-Ir atoms (Ir_(cus)) are separated by rows of bridging-O atoms (O_(br)) that run parallel to the [001] direction. The Ir_(cus) and O_(br) atoms each lack a bonding partner compared with the bulk and expose single coordination vacancies. Based the IrO₂(110) unit cell, the areal densities of Ir_(cus) atoms and O_(br) atoms would each equal to 37% of the surface atom density of Ir(100). Because the cus-metal atoms are active adsorption sites, adsorbate coverages were specified in units of ML (monolayer), where 1 ML is equal to the density of Ir_(cus) atoms on the IrO₂(110) surface.

Model representations of the stoichiometric IrO₂(110) surface with Ir_(cus), O_(br) and 3f-O atoms labeled are shown in FIG. 1A. The Ir and O atoms are represented as blue and red spheres, respectively. FIG. 1B provides a LEED pattern obtained from an ˜3.5 nm IrO₂(110) film grown on Ir(100) by oxidizing in 5 Torr of O₂ at 775 K. The orange circles mark the LEED spot positions from the Ir(100) substrate and the light blue and pink spots represent reciprocal space points computed for two orientations of the rectangular IrO₂(110) unit cell with dimensions of 1.16 a×2.34 a where a is the Ir(100) lattice constant.

Oxidation of Ir(100) at 775 K and an O₂ partial pressure of 5 Torr produced a high-quality IrO₂(110) layer that exposed the stoichiometric termination. The experimental methods used herein are described in the Materials and Methods section. A representative LEED pattern obtained after oxidizing Ir(100) to form the IrO₂(110) layer (FIG. 1B) agreed quantitatively with that simulated for two rotational domains of the IrO₂(110) structure with unit cell dimensions of 3.16 Å by 6.36 Å. The IrO₂(110) lattice vectors align with the high-symmetry [001] and [110] directions of the Ir(100) growth substrate. The absence of Ir(100) diffraction spots in the LEED pattern is consistent with the presence of a conformal IrO₂(110) layer that is thick enough to completely attenuate the elastic scattering of electrons from the underlying Ir(100) substrate. The IrO₂(110) layer is stable to a temperature of ˜725 K but thermally decomposes at higher temperature. Quantification of the O₂ TPD feature centered at ˜900 K (FIGS. 5AB) allows an estimate that the IrO₂(110) film contained 46 ML of O-atoms and was ˜3.5 nm thick. FIG. 5A provides schematic models of the IrO₂(110) layer. Perspective view (top) and layer stacking along the [110] direction (bottom) are both shown. Each layer consists of 2 ML Ir and 4 ML O atoms where 1 ML is equal to the cus-Ir density. FIG. 5B is an O₂ TPD spectrum obtained by thermally decomposing the IrO₂(110) layer in UHV. It was confirmed that the IrO₂(110) layer was stoichiometrically terminated by using TPRS to characterize the adsorption behavior of O₂ and H₂O (FIGS. 6A-B).

The TPRS traces obtained after adsorbing low and high CH₄ coverages (0.10 and 0.53 ML, FIGS. 2A and 2B) on the IrO₂(110) surface at 88 K revealed that a large fraction of the adsorbed CH₄ oxidizes to CO, CO₂, and H₂O during heating, with the CO and CO₂ products desorbing between ˜400 and 600 K. The small CO TPRS feature near 250 K is consistent with a small quantity of CO that adsorbed from the vacuum background. The H₂O TPRS feature is broader than the CO and CO₂ features and spanned a range from ˜400 to 750 K. Desorption of CH₄ also occurred with separate TPRS peaks centered at ˜130 K and 515 K. The low-temperature TPRS peak is characteristic of the desorption of a CH₄ σ-complex that is bound strongly to the IrO₂(110) surface (˜34 to 43 kJ/mol). FIGS. 2A-B show TPRS spectra of CH₄, H₂O, CO and CO₂ obtained after adsorbing CH₄ on IrO₂(110) at 88 K to generate coverages of 0.10 ML (FIG. 2A) and 0.53 ML (FIG. 2B). In contrast, Redhead analysis of the high-temperature CH₄ peak suggests an activation energy for desorption (130-140 kJ/mol) that is far too high for this TPRS peak to originate from molecularly adsorbed CH₄. The high temperature CH₄ peak results from the recombination of adsorbed CH₃ groups and H atoms were confirmed by performing experiments of CD₄ adsorption onto an as-prepared IrO₂(110) film that was covered by a small amount of residual H-atoms (see SM for details). Based on the detected mass fragments, these measurements demonstrate that only CD₄ desorbs in the TPRS peak at 130 K while both CD₄ and CD₃H contribute to the TPRS peak at 510 K (FIGS. 7A-C). FIGS. 7A-B show TPRS spectra of CD₄ ⁺ (m/z=20) and CD₃ ⁺ (m/z=18), and FIG. 7C is a TPRS spectra of CHD₃ ⁺ (m/z=19) and CHD₂ ⁺ (m/z=17) obtained after adsorbing CD₄ on IrO₂(110) at 88 K. The high temperature shoulder (575 K to 800 K) of each spectrum are water product/fragment, which are labeled based on their m/z ratios. Below, it is shown that energies derived from DFT calculations also support the described assignments of the low and high temperature CH₄ TPRS peaks to molecular vs. recombinative desorption processes.

The TPRS data demonstrates that a large quantity of CH₄ undergoes C—H bond activation on the IrO₂(110) surface, with the dissociated products oxidizing to CO, CO₂ and H₂O and also recombining to CH₄ at higher temperature. The observed reactivity is consistent with a precursor-mediated mechanism wherein a fraction of the adsorbed CH₄ σ-complexes undergo C—H bond cleavage rather than desorbing, and the resulting CH₃ and OH fragments react during continued heating. An implication is that CH₄ C—H bond cleavage occurred readily on IrO₂(110) at temperatures as low as ˜150 K and even lower, i.e., below the temperatures at which the adsorbed complexes desorb during TPRS. There are no other known materials that exhibit such high activity toward promoting CH₄ C—H bond cleavage.

The TPRS yields of reacted vs. unreacted CH₄ on IrO₂(110) as a function of the CH₄ exposure performed at 88 K are shown in FIG. 2C. The yield of reacted CH₄ is defined here as the sum of the yields of CO, CO₂ and CH₄ that desorbed in the TPRS peak at ˜515 K, which are attributed to recombinatively desorbed CH₄, and the yield of unreacted CH₄ (“molecular”) as equal to the amount of CH₄ that desorbed in the low temperature TPRS peak. The yield of reacted CH₄ increased as the exposure initially increased, while the yield of unreacted CH₄ remained quite low. More than 85% of the adsorbed CH₄ reacted during TPRS at total CH₄ coverages below ˜0.30 ML. The yield of reacted CH₄ plateaued at ˜0.30 ML after ˜1.0 ML exposure. The yield of unreacted CH₄ increased only after the yield of reacted CH₄ had nearly saturated, demonstrating that the IrO₂(110) surface is highly reactive toward CH₄ at CH₄ coverages below about 0.33 ML. The total CH₄ coverage saturated at a value near 0.54 ML. A similar saturation coverage of CH₄ on the RuO₂(110) surface at 80 K has been reported in previous research (19). Thus, ˜55% of the adsorbed CH₄ on IrO₂(110) reacted during TPRS when the CH₄ layer is saturated. The large quantity of CH₄ that reacts during TPRS is consistent with C—H bond activation occurring on the Ir_(cus) and O_(br) sites that are present on the crystalline terraces of the IrO₂(110) surface. TPRS yields of molecularly desorbed and reacted methane and the total CH₄ TPRS yield as a function of the CH₄ exposure to the surface are shown in FIG. 2C. The total yield of reacted CH₄, yields of oxidized CH₄ (CO+CO₂) and recombinatively-desorbed CH₄ and the total amount of O-atoms removed from the oxide during TPRS as a function of the CH₄exposure is shown in FIG. 2D.

The change in TPRS yields of reaction products with CH₄ exposure are shown in FIG. 2D. Dissociated CH₄ preferentially oxidized to CO and CO₂ during TPRS for initial CH₄ coverages below ˜0.16 ML, but the selectivity toward oxidation over recombination continuously decreased as the total reaction yield increased. The oxidized and recombined yields became equal at ˜0.16 ML when the total reaction yield reached saturation at 0.30 ML. The CO₂ yield was ˜1.4 times higher than the CO yield at all CH₄ coverages, with yields reaching saturation values of 0.09 and 0.065 ML. Lastly, CH₄ oxidation during TPRS removed ˜0.50 ML of O-atoms from the IrO₂(110) surface when the yield of oxidized products saturated, which is equal to half of the O_(br) site concentration. The stoichiometric constraints as well as a relatively high stability of HO_(br) groups likely play an important role in determining the total reaction yield as well as the branching between CH₄ oxidation and recombination during TPRS.

Experimental estimates of the CH₄ dissociation probability obtained as a function of the surface temperature were evaluated using a kinetic model for the precursor-mediated dissociation of CH₄. The dissociation of an alkane from a molecularly-adsorbed precursor state was represented by the following kinetic scheme (3, 6).

$\begin{matrix} {{{RH}(g)}\underset{k_{d}}{\overset{\mspace{14mu}{\xi\; F}\mspace{14mu}}{\rightleftarrows}}{{{{RH}({ad})}\overset{k_{r}}{\longrightarrow}{R({ad})}} + {H({ad})}}} & (1) \end{matrix}$ where RH represents an alkane molecule, ξ is the probability for molecular adsorption, F is the incident flux of gaseous RH at the surface, and k_(d) and k_(r) are rate coefficients for desorption and dissociation (“reaction”) via C—H bond cleavage of the molecularly-adsorbed RH σ-complex. The kinetic scheme treats the reaction step as irreversible and is applicable at a temperature below that at which recombination becomes kinetically relevant. It is assumed that the probability for CH₄ to adsorb molecularly into the σ-complex state is unity and independent of the surface temperature, because molecular adsorption is non-activated and the impingent CH₄ molecules have kinetic energies that are much lower than the strength of the molecule-surface interaction (˜2.5 vs. 40 kJ/mol) (3). Molecular beam scattering experiments show that probabilities for non-activated adsorption are nearly independent of the surface temperature (26). The following expression for the dissociative chemisorption probability in the limit of zero coverage (S₀) was derived by applying the steady-state approximation to the rate of formation of molecularly-adsorbed alkanes:

$\begin{matrix} {S_{0} = \frac{\xi\; k_{r}}{k_{r} + k_{d}}} & (2) \end{matrix}$ If it is assumed that the Arrhenius equation describes the temperature dependence of each rate coefficient, then:

$\begin{matrix} {{\ln\left( {\frac{\xi}{S_{0}} - 1} \right)} = {{\ln\left( \frac{v_{d}}{v_{r}} \right)} - \frac{\left( {E_{d} - E_{r}} \right)}{{RT}_{s}}}} & (3) \end{matrix}$ where v_(j) and E_(j) represent the prefactor and activation energy for reaction j, and T_(s) is the surface temperature. Thus, if CH₄ dissociates on IrO₂(110) by a precursor-mediated mechanism, then a plot of

${\ln\left( {\frac{\xi}{S_{0}} - 1} \right)}\mspace{14mu}{{vs}.\mspace{14mu}\frac{1}{T_{s}}}$ will be linear and the Arrhenius construction will provide values for the apparent pre-factor

$\frac{v_{r}}{v_{d}}$ and activation energy E_(r)−E_(d) for initial C—H bond cleavage. To obtain estimates of the CH₄ dissociation probability, the TPRS yields of reacted CH₄ (CO, CO₂, and recombinatively desorbed CH₄) was measured as a function of the CH₄ exposure at several fixed surface temperatures between 175 and 300 K. 175 K was selected as the lower limit because this temperature lies above the trailing edge of the low-temperature CH₄ TPD peak. Because molecularly adsorbed CH₄ accumulates negligibly above 175 K, the TPRS yields of CO, CO₂, and recombinatively-desorbed CH₄ were equal to the amount of CH₄ that dissociated on the surface during the CH₄ exposures. 300 K was selected as the upper limit to minimize the loss of surface oxygen via product desorption and thus avoid possible changes in surface reactivity that could occur during the CH₄ exposures caused by partial reduction of the oxide surface. The TPRS results also showed that recombination of CHs and H atoms was negligible below 300 K and could be ignored in the analysis.

CH₄ reaction yields were measured as a function of the exposure at several T_(s); exposures were short to maintain low coverages of the reaction products. Each isotherm of the reaction yield vs. exposure (FIG. 3A) was well-approximated as linear, with the slopes decreasing with increasing T_(s). The linear behavior was expected because the probability for dissociative chemisorption is approximately independent of the adsorbate coverage at low coverage. In this limit, the coverage of dissociated CH₄ [R] is given by the equation, [R]=S₀Ft where the CH₄ exposure is equal to the product of the exposure time t and the incident flux F, which was estimated as 1.1×10⁻² ML/s. The slope of the initial portion of an isotherm is thus equal to the initial dissociation probability of CH₄ on IrO₂(110) for the T_(s) at which the exposure was conducted. Our estimates of the initial dissociation probability S₀ at various T_(s) of CH₄ on IrO₂(110) are plotted in FIG. 3B along with the curve that represents the expression S₀ (T_(s)) determined from our kinetic analysis, as discussed below. The initial dissociation probability decreased from about 80% to 19% with increasing T_(s) from 175 to 300 K. The IrO₂(110) surface was remarkably active toward promoting C—H bond cleavage: 80% of the CH₄ molecules that collide with the clean surface underwent C—H bond scission at a surface temperature of only 175 K. The decrease in initial dissociation probability with increasing T_(s) is characteristic of a facile precursor-mediated mechanism.

The excellent linear fit of

${\ln\left( {\frac{\xi}{S_{0}} - 1} \right)}\mspace{14mu}{{vs}.\mspace{14mu}\frac{1}{T_{s}}}$ (FIG. 3C) further supports the conclusion that CH₄ dissociation on IrO₂(110) occurs by a precursor-mediated mechanism, with an apparent pre-factor of 6.2×10⁻³ and an activation energy of −9.5 kJ/mol (negative relative to the gas-phase reference). It is noted that the apparent activation energy and pre-factor for reaction depend only weakly on the value of ξ used in the analysis. Analysis of the low temperature CH₄ TPRS peak suggests a binding energy of ˜38 kJ/mol at low CH₄ coverage, from which a value of 28.5 kJ/mol for the activation energy of C—H bond cleavage of the CH₄ σ-complex on IrO₂(110) can be estimated. For comparison, the reaction barrier that was estimated for CH₄ activation on IrO₂(110) is roughly half of that for CH₄ activation on PdO(101) (28.5 vs. 56 kJ/mol).

The energy diagrams were computed for the formation and dissociation of a CH₄ σ-complex on IrO₂(110) as well as images of the initial, transition and final states are shown in FIG. 4. The shown results were computed using conventional DFT (black) and DFT-D3 (red) as well as images of the initial, transition and final states. The energies were determined using conventional DFT and the dispersion-corrected DFT-D3 method (27), both employing the PBE exchange-correlation functional. Details of the DFT calculations can be found in the Materials and Methods below, along with CH₄ adsorption energies obtained using several DFT functionals that incorporate dispersion. The calculations predict a facile pathway for C—H bond cleavage of CH₄ on IrO₂(110). A plot of the charge-density difference (FIG. 8) shows that the CH₄ molecule forms a strongly-bound σ-complex on IrO₂(110) by adopting an η² configuration and datively bonding with a single Ir_(cus) atom. The image was generated using VESTA (41) and shows the CH4 molecule, an Ircus atom and an O-atom underneath the Ircus atom. The inset shows a representation of the CH₄/IrO₂(110) structure. Back donation of charge from the Ir_(cus) atom to CH₄ weakens the C—H bond and promotes C—H bond cleavage. In the C—H bond cleavage step, the CH₄ complex transfers an H-atom to a neighboring O_(br) atom, resulting in CH₃—Ir_(cus) and HO_(br) moieties. Both the DFT and DFT-D3 calculations predict that the energy barrier for dissociation lies below the gas-phase energy level so that C—H bond cleavage is energetically preferred over desorption of the adsorbed CH₄ complex.

The dispersion-correction included in the DFT-D3 calculations increased the binding energies computed for each adsorbed state compared with the results of the DFT-PBE calculations. Because the enhancement is similar for the initial state and the transition state, both the DFT and DFT-D3 calculations predict similar values for the C—H bond cleavage barrier relative to the initial adsorbed state (E_(r)˜19 vs. 24 kJ/mol), where these values agreed reasonably well with the value of E_(r)=28.5 kJ/mol estimated from our experimental data. Also, our experimental estimates of the binding energy and apparent dissociation barrier for the adsorbed CH₄ complex agreed well with the values computed using DFT-PBE. From the experimental data, values of E_(d)˜38 kJ/mol and E_(r)−E_(d)=−9.5 kJ/mol were estimated, where these values agree to within better than 2.5 kJ/mol of the values predicted by DFT-PBE (E_(d)=35.7 kJ/mol; E_(r)−E_(d)=−11.6 kJ/mol). The rate coefficient governing the recombinative desorption of CH₄ via the reaction CH₃+HO_(br)→CH₄(g)+O_(br) is approximately equal to the rate coefficient for only the recombination step that produces the adsorbed CH₄ σ-complex when the temperature is sufficiently high. The results of both the DFT-PBE and DFT-D3 calculations predict an energy barrier of about 129 kJ/mol for this recombination step (FIG. 4). For desorption pre-factors of 10¹² and 10¹³ s⁻¹, the CH₄ TPRS peak observed at 515 K likely corresponds to activation energies of 130 and 140 kJ/mol, respectively.

The facile activation of CH₄ on cus-Ir/O surface pairs may provide opportunities for developing catalysts and catalytic processes that can promote efficient and selective methane functionalization. For example, certain co-reactants may directly react with CH₄-derived fragments on IrO₂(110) to produce value-added compounds. It may also be possible to modify the IrO₂(110) surface to limit its oxidizing power and/or incorporate cus-Ir/O surface pairs into other materials that promote more desirable methane chemistries, such as conversion to organic oxygenates or higher hydrocarbons.

Materials and Methods

Experimental Setup for UHV Surface Analysis Chamber (27) (27)

Experiments for this study were performed in an apparatus consisting of an UHV surface analysis chamber with an isolatable reaction cell that is attached to the bottom of the chamber. The UHV chamber contains a four-grid retarding field analyzer for surface characterization using low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) and a shielded quadrupole mass spectrometer used for TPRS. The Ir(100) crystal employed in this study is a circular disk (9 mm×1 mm) that is attached to a liquid-nitrogen cooled, copper sample holder by 0.015″ W wires that are secured to the edge of crystal. A type K thermocouple was spot-welded to the backside of the crystal for temperature measurements. Resistive heating, controlled using a PID controller that varies the output of a programmable DC power supply, supports linearly ramping from 80 K to 1500 K or maintaining the sample temperature. Sample cleaning consisted of cycles of Ar⁺ sputtering (2000 eV, 15 mA) at 1000 K, followed by annealing at 1500 K for several minutes. The sample was subsequently exposed to 5×10⁻⁷ Torr of O₂ at 900 K to remove surface carbon, followed by flashing to 1500 K to remove final traces of oxygen. The Ir(100) sample was considered to be clean when a sharp (5×1) LEED pattern was obtained consistent with the surface reconstruction of clean Ir(100), no impurities were detected using AES and negligible CO and CO₂ production was observed during flash desorption after adsorbing oxygen.

Experimental Setup for Ambient-pressure Reaction Cell

IrO₂(110) films were generated by oxidizing Ir(100) in the isolated reaction cell at an O₂ partial pressure of 5 Torr and a surface temperature of 775 K. The reaction cell is a six-way cross that is separated from the UHV chamber by a gate valve and a differentially-pumped tube that contains two spring-loaded Teflon sliding seals positioned at the top and bottom of the tube. The sample holder mounts onto the bottom of a well-polished stainless steel tube with a cross-sectional area that is larger than the holder. As the sample is translated downward into the reaction cell, the sliding seals grip the sample probe tube and establish seals that isolate the reaction cell from the UHV chamber. This sealing mechanism allows exposure of the Ir(100) sample to elevated gas pressures in the reaction cell at variable sample temperatures, while maintaining UHV in the analysis chamber. After completing the desired O₂ exposure, the reaction cell was evacuated and the sample translated back to the UHV chamber for surface characterization. The as-prepared IrO₂(110) film contains a small quantity (<0.10 ML) of H-atoms that likely adsorbed from the background in the reaction cell after the high-pressure O₂ exposure. The concentration of residual H-atoms was estimated by saturating the as-prepared oxide surface with O₂ and monitoring the amount of H₂O that desorbs during TPRS. The residual H-atoms can be removed by exposing the film to O₂ in the UHV chamber while cycling the surface temperature between 300 K and 650 K.

Temperature Programmed Reaction Spectroscopy (TPRS) Measurements

The reactivity of CH₄ (Airgas, 99.999%) on the s-IrO₂(110) (“s=stoichiometric”) surface was studied using TPRS. Methane was delivered to the sample from a calibrated beam doser at an incident flux of approximately 0.011 ML/s with the sample-to-doser distance set to about 15 mm to ensure uniform impingement of methane across the sample surface. TPRS spectra were collected after methane exposures by positioning the sample in front of a shielded mass spectrometer at a distance of about 5 mm and then heating at a constant rate of 1 K/s until the sample temperature reached 700 K. Initially, a wide range of desorbing species was monitored to identify the main products that are generated from reactions of methane on s-IrO₂(110), and it was found that the only species desorbing from the CH₄-exposed s-IrO₂(110) sample are methane, water, CO and CO₂. After each TPRS experiment, the surface was exposed to 24 L of O₂ supplied through a tube doser while cycling the surface temperature between 300 and 650 K. Consecutive TPRS experiments demonstrate that this surface cleaning/restoration procedure fully restores the surface reactivity toward CH₄. However, caution must be made when performing TPRS to temperatures higher than 725 K, which is the onset of oxide decomposition. The data shows that the surface reactivity diminishes after repeated CH₄ adsorption at 88 K and ramping the temperature to 800 K during the TPRS measurements, even when performing the surface cleaning/restoration treatment between experiments. Repeating the same CH₄ exposure on oxide films generated on different days gave identical CO and CO₂ desorption features in the TPRS data. Such behavior supports the idea that the activation of CH₄ occurs on terraces rather than defects, because the latter would likely exhibit variability in concentration and distribution and thus cause variations in the observed reactivity.

Measurement of Product Yields

Atomic oxygen coverages were estimated by scaling integrated O₂ TPD spectra with those obtained from a saturated (2×1) layer containing 0.50 ML of O-atoms, prepared by exposing the Ir(100)−(1×1) surface to O₂ in UHV (28). To estimate CO desorption yields, integrated CO desorption spectra were scaled by an integrated TPD spectrum collected from a saturated c(2×2) layer containing 0.50 ML of CO that were prepared by adsorbing CO to saturation on Ir(100)−(1×1) at 300 K (29, 30). TPRS experiments of CO oxidation on O-covered Ir(100) to estimate the CO₂ desorption yields were performed. In these experiments, O₂, CO₂ and CO TPRS spectra were collected after exposing a (2×1)−O layer to a saturation dose of CO and it was then assumed that the CO₂ TPRS yield is equal to the difference between the initial (0.50 ML) and final coverages of oxygen where the final oxygen coverage is determined from the O₂ TPRS yield. Lastly, the H₂O and CH₄ TPRS yields were estimated by scaling the intensity-to-coverage conversion factors determined for CO, CO₂ and O₂ with relative sensitivity factors reported for the mass spectrometric detection of these gases.

Computational Details

All plane wave DFT calculations were performed using the projector augmented wave pseudopotentials (31) provided in the Vienna ab initio simulation package (VASP) (32, 33). The Ir 5d and 6s states are treated as valence electrons, but the adsorption minima for CH₄ on IrO₂(110) have also been tested using a pseudopotential that includes the Ir 5s and 5p states as valence electrons and a negligible change in the adsorption energy found. The Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional (34) was used with a plane wave expansion cutoff of 400 eV. Dispersion interactions are modeled using the DFT-D3 method developed by Grimme et al. (29). This method provides accurate estimates of the adsorption energies of n-alkanes on PdO(101) (11) and RuO₂(110) (19) in comparison with TPD-derived values. The bulk structure of IrO₂ was generated by using the PBE functional and the lattice constants a and c are predicted to be 4.54 Å and 3.19 Å, respectively. These DFT results are consistent with experimental values of a=4.50 Å and c=3.15 Å (12). Based on the bulk structure from DFT-PBE, the IrO₂(110) surface was generated to perform DFT-PBE and DFT+D3 calculations. Four layers were employed to model the IrO₂(110) film, resulting in an ˜12 Å thick slab. The bottom two layers are fixed, but all other lattice atoms are allowed to relax during the calculations until the forces are less than 0.03 eV/Å. A vacuum spacing of ˜25 Å was included, which is sufficient to reduce the periodic interaction in the surface normal direction. In terms of system size, a 1×4 unit cell with a corresponding 4×2×1 Monkhorst-Pack k-point mesh is used.

In the present study, the binding energy, E_(b), of an adsorbed CH₄ molecule on the surface is defined using the expression, E _(b)=(E _(CH) ₄ +E _(surf))−E _(CH) ₄ _(/surf)  (4) where E_(CH) ₄ _(/surf) is the energy of the initial state containing the adsorbed CH₄ molecule, E_(surf) is the energy of the bare surface, and E_(CH) ₄ is the energy of an isolated CH₄ molecule in the gas phase. All reported binding energies are corrected for zero-point vibrational energy. From equation 4, a large positive value for the binding energy indicates a high stability of the adsorbed CH₄ molecule under consideration. Barriers of C—H bond cleavage of adsorbed CH₄ on the IrO₂(110) surface were evaluated using the climbing nudged elastic band (cNEB) method (35). All of the calculations were performed for a single methane molecule adsorbed within the 1×4 surface model of IrO₂(110), and corresponds to a methane coverage equal to 25% of the Ir_(cus) density. Thermal Decomposition of the IrO₂(110) Film

FIG. 5A shows ball and stick models of rutile IrO₂ shown in a perspective view and also a side view perpendicular to the [110] direction. Each repeat unit of the oxide along the [110] direction contains 4 ML of oxygen atoms and the spacing between these IrO₂(110) layers is equal to 3.2 Å. FIG. 5B shows an O₂ TPD spectrum obtained during thermal decomposition of the IrO₂(110) film that was grown on Ir(100) by oxidizing in 5 Torr of O₂ at 775 K for a duration of 10 minutes. Decomposition of the IrO₂(110) film produces an O₂ TPD feature centered at ˜900 K. From quantification of the O₂ desorption yield, it is estimated that the IrO₂(110) film contained 46 ML of oxygen atoms. This quantity of oxygen atoms corresponds to an IrO₂(110) film thickness of 3.5 nm, based on the structure of rutile IrO₂ (FIG. 5A).

O₂ and H₂O Adsorption on the IrO₂(110) Film

FIG. 6A shows an O₂ TPD spectrum obtained after exposing the IrO₂(110) film to a saturation dose of O₂ at a surface temperature of 88 K. The O₂ TPD trace exhibits an intense feature below 300 K as well as a broad feature centered at ˜515 K that are consistent with the molecular desorption of O₂ and the recombination of O-atoms from the Ir_(cus) sites of IrO₂(110), respectively. Oxygen also dissociatively chemisorbs on the s-RuO₂(110) surface and recombinatively desorbs in a TPD feature between ˜300 and 500 K (36). It is estimated that a total of 0.70 ML of oxygen desorbs in the TPRS experiment, with ˜0.38 ML desorbing in the high temperature peak. The total oxygen coverage is equal to a large fraction of the density of the Ir_(cus) atoms on the s-IrO₂(110) surface. The saturation coverage of oxygen on the Ir_(cus) sites is likely to be less than 1 ML, given that the random adsorption of dimers on a line of sites reaches a theoretical jamming coverage of 0.86 ML.

FIG. 6B shows a H₂O TPD spectrum obtained after adsorbing 2.6 ML of H₂O on the IrO₂(110) film at 88 K. The H₂O desorption trace exhibits a broad feature centered at ˜500 K and an intense peak at 170 K with a shoulder at ˜200 K. An estimated 1.0 ML of water desorbs in the high temperature peak and this feature is attributed to H₂O species that are adsorbed on the Ir_(cus) sites of IrO₂(110). The sharp peak at ˜170 K arises from water adsorbed in a multilayer state and the shoulder at 200 K is consistent with H₂O adsorbed on O_(br) sites. The H₂O TPD spectrum obtained from IrO₂(110) is similar to that reported for water-covered RuO2(110) (37). The adsorption behavior of O₂ and H₂O provides additional evidence that the IrO₂(110) film grown in the disclosed experiments is stoichiometrically-terminated.

Adsorption of CD₄ on the IrO₂(110) Film

TPRS experiments were performed with adsorbed CD₄ to confirm that the low and high temperature TPRS peaks of methane originate from molecular vs. recombinative desorption processes. The reactivity of CD₄ was lower than CH₄ but the kinetic isotope effect is not further discussed in this study. FIGS. 7A-C show TPRS spectra of the 17 to 20 amu mass fragments obtained after saturating an as-prepared s-IrO₂(110) film with CD₄ at 88 K where the as-prepared film was covered by between 0.05 to 0.10 ML of H-atoms that adsorbed from the background (see Materials and Methods). The mass fragments to specific methane-derived ions were assigned as follows: 17 amu (CHD₂ ⁺), 18 amu (CD₃ ⁺), 19 amu (CD₃H⁺) and 20 amu (CD₄ ⁺). The 20 amu fragment arises from CD₄ while the 17 and 19 amu fragments originate from partially hydrogenated methane. The 18 and 20 amu TPRS traces exhibit an intense peak at 128 K (FIG. 7A), consistent with molecularly-adsorbed CD₄. In contrast, the 17 and 19 amu traces exhibit negligible intensity in the low temperature peak, while the 17, 18, 19 and 20 amu TPRS traces all exhibit a peak centered at ˜530 K. Notably, the H₂O, HDO and D₂O also contribute to the 17 to 20 amu TPRS features observed above ˜400 K, and that methane desorption generates the distinct feature centered at 530 K. These results demonstrate that only CD₄ desorbs in the TPRS peak near 130 K, whereas CD₄ as well as CD₃H desorb in the peak at 530 K, thus supporting the assignment of the low and high temperature TPRS peaks to molecularly-adsorbed methane vs. the recombination of adsorbed methyl groups and H-atoms.

Quantitative Accuracy of the DFT Results

Comparison with the experimental data shows that DFT-D3 overestimates the binding of the CH₄ σ-complex on IrO₂(110). In contrast, it was previously found that DFT-D3 quantitatively reproduces the binding energies and apparent reaction barriers of light alkanes on the PdO(101) and RuO2(110) surfaces (11, 12, 19, 20). Because methane experiences dispersion interactions with the IrO₂(110) surface and yet such interactions are omitted from DFT-PBE, it is concluded here that the good quantitative agreement between the energies determined from experiment and DFT-PBE calculations is coincidental and that the DFT-PBE calculations actually overestimate the binding energy resulting from the covalent dative bonding between the CH₄ molecule and the IrO₂(110) surface. Recent DFT calculations using the optB88-vdw functional also overestimate the CH₄ binding energies on IrO₂(110) (18), by a similar amount as the DFT-D3 calculations. The adsorption energy of CH₄ on IrO₂(110) for several functionals has been tested, including the hybrid HSE06 and PBEO functionals and also functionals that incorporate dispersion (see results summarized in Table 1).

TABLE 1 E_(ads) (kJ/mol) for CH₄ on IrO₂(110) using various XC functionals with and without dispersion. Values given in parentheses include zero point corrections. XC functionals without dispersion Static Static Functional PBE RPBE revPBE HSE06 PBE0 E_(ads) 39.6 (35.7) 11.6 (6.3) 13.5 37.6 45.3 (kJ/mol) XC functionals with dispersion PBE- optPBE- optB88- opt86b- Functional D3 TS vdw vdw vdw DF2-vdw E_(ads) 70.4 67.5 64.6 72.4 78.1 48.2 (43.5) (kJ/mol)

The RPBE and revPBE functionals underestimate the CH₄ binding energy on IrO₂(110) by 20-25 kJ/mol in comparison to PBE, and similar findings have been reported for several other molecules (38). It is clear that while the RPBE functional has been reported to be more accurate for some small molecules on metal surfaces, it fails to capture the strong contribution to the σ-bonding for CH₄ on IrO₂(110). For the hybrid-functional DFT calculations, only static single point calculations using the adsorption configuration from DFT-PBE were performed. The hybrid functionals do not dramatically change the adsorption energy in comparison to the PBE functional and full relaxation will lead to more strongly-bound CH₄ than found with PBE. All of the XC functionals with dispersion give similar results to PBE-D3, reinforcing the observation above that the dispersion contributions are being captured accurately. The one exception to this finding is the DF2-vdw functional, which incorporates dispersion into the revPBE family of functionals (39). Because the revPBE functional underestimates adsorption energy, the inclusion of dispersion results in an adsorption energy closer to the experimental value (and DFT-PBE with no dispersion). However, this agreement is likely fortuitous and the DF2-vdw functional is not accurately capturing the σ-bonding in this system. To test the DF2-vdw functional further, a NEB calculation was performed using this functional and find a ZPC-value of 54.9 kJ/mol for the C—H bond cleavage barrier. Combined with the ZPC E_(ads) value of 43.5 kJ/mol, this results in a positive apparent barrier of 11.4 kJ/mol, conflicting with the experimental result. The source of this overestimation of the barrier to C—H bond activation is likely due to the failure of the revPBE functional to capture the σ-bonding, which leads to back-donation charge transfer that weakens the C—H bond and facilitates C—H activation.

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Ratios, concentrations, amounts, and other numerical data may be expressed in a range format. It is to be understood that such a range format is used for convenience and brevity, and should be interpreted in a flexible manner to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. To illustrate, a concentration range of “about 0.1% to about 5%” should be interpreted to include not only the explicitly recited concentration of about 0.1% to about 5%, but also include individual concentrations (e.g., 1%, 2%, 3%, and 4%) and the sub-ranges (e.g., 0.5%, 1.1%, 2.2%, 3.3%, and 4.4%) within the indicated range. In an embodiment, the term “about” can include traditional rounding according to significant figure of the numerical value. In addition, the phrase “about ‘x’ to ‘y’” includes “about ‘x’ to about ‘y’”.

Unless defined otherwise, all technical and scientific terms used have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Although any methods and materials similar or equivalent to those described can also be used in the practice or testing of the present disclosure, the preferred methods and materials are now described.

Embodiments of the present disclosure will employ, unless otherwise indicated, techniques of separating, testing, and constructing materials, which are within the skill of the art. Such techniques are explained fully in the literature.

It should be emphasized that the above-described embodiments are merely examples of possible implementations. Many variations and modifications may be made to the above-described embodiments without departing from the principles of the present disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims. 

At least the following is claimed:
 1. A catalyst comprising: a IrO₂(110) substrate having a rutile IrO₂(110) surface having exposed cus-Ir atom sites.
 2. The catalyst of claim 1, wherein a stoichiometric termination of rutile IrO₂(110) has a rectangular unit cell with dimensions of 3.16×6.36 Å with corresponding lattice vectors aligned along [001] and [110] crystallographic directions, respectively.
 3. The catalyst of claim 1, wherein IrO₂(110) surface has rows of cus-Ir atoms separated by rows of bridging-O atoms that run parallel to a [001] direction.
 4. The catalyst of claim 3, wherein cus-Ir atoms and bridging-O atoms each lack a bonding partner compared with a bulk IrO₂ surface, wherein the lack of a bonding partner exposes single coordination vacancies.
 5. The catalyst of claim 2, wherein each IrO₂(110) unit cell has an areal density of cus-Ir atoms and bridging-O atoms that each equal about 34 to 40% of a surface atom density of an Ir(100) surface.
 6. The catalyst of claim 2, wherein each IrO₂(110) unit cell has an areal density of cus-Ir atoms and bridging-O atoms that each equal 37% of a surface atom density of an Ir(100) surface.
 7. A method of making an IrO₂(110) surface, comprising: oxidizing Ir(100) substrate at about 725 to 875 K and a O₂ partial pressure of about 0.5 to 100 Torr; evacuating O₂ from the chamber until the pressure reaches less than 10⁻⁷ Torr with the sample held at 600 to 650 K; and forming a rutile IrO₂(110) surface having exposed cus-Ir atom sites.
 8. A method of making a product from CH₄, comprising: exposing a catalyst comprising an IrO₂ substrate having a rutile IrO₂(110) surface having exposed cus-Ir atom sites and a CH₄ gas to one another; and forming one or more products.
 9. The method of claim 8, wherein the product is selected from the group consisting of: CH₃OH, CH₂O₁, C₂H₄, and a combination thereof.
 10. A system of activating CH₄, comprising: a first device for introducing CH₄ to a catalyst comprising an IrO₂ substrate having a rutile IrO₂(110) surface having exposed cus-Ir atom sites; a second device for collecting one or more products of the catalytic reaction of CH₄.
 11. The system of claim 10, wherein the CH₄ is introduced to the substrate at about 150 to 300 K.
 12. The system of claim 10, wherein the product is selected from the group consisting of: CH₃OH, CH₂O, C₂H₄, and a combination thereof.
 13. The system of claim 10, wherein a stoichiometric termination of rutile IrO₂(110) has a rectangular unit cell with dimensions of 3.16×6.36 Å with corresponding lattice vectors aligned along [001] and [110] crystallographic directions, respectively.
 14. The system of claim 10, wherein IrO₂(110) surface has rows of cus-Ir atoms separated by rows of bridging-O atoms that run parallel to a [001] direction.
 15. The system of claim 14, wherein cus-Ir atoms and bridging-O atoms each lack a bonding partner compared with a bulk IrO₂ surface, wherein the lack of a bonding partner exposes single coordination vacancies.
 16. The system of claim 13, wherein each IrO₂(110) unit cell has an areal density of cus-Ir atoms and bridging-O atoms that each equal 37% of a surface atom density of an Ir(100) surface. 